The present invention relates to a process for preparing 2,2',4,4',6,6'-hexanitrostilbene (HNS) through oxidation of 2,2',4,4',6,6'-hexanitrobibenzyl (HNBB) with oxygen containing gas in the presence of a catalyst.
2,2',4,4',6,6'-Hexanitrostilbene (HNS) is a thermally stable explosive. HNS has also been used as a nucleating agent for promoting a desired mode of crystallization of 2,4,6-trinitrotoluene (TNT) in melt-cast TNT explosives.
HNS has been prepared in rather low yields by oxidation of TNT. Thus, Shipp, U.S. Pat. No. 3,505,413, NOLTR (U.S. Naval Ordnance Laboratory Technical Report) 64-34 (1964), and Shipp and Kaplan, Journal of Organic Chemistry, 31,857 (1966), disclose a process which comprises reacting TNT with sodium hypochlorite in a solvent mixture consisting of two parts tetrahydrofuran (THF) and one part methanol by volume to produce crude HNS in yields of 40-45% of theory. The crude HNS, which contains substantial amounts of coprecipitated impurities, is purified by extraction with hot acetone. Shipp and Kaplan disclose that TNT can be converted to HNBB or HNS with sodium hypochlorite under various conditions, with a yield of 79% HNBB from TNT. No procedure for preparing HNS from HNBB is disclosed.
The British Patent application No. 76/2501 to Salter et al, Jan. 22, 1976, discloses a process wherein TNT is reacted in THF-methanol solution with sodium hypochlorite at approximately 10.degree.-20.degree. C., with subsequent addition of an aqueous solution of an organic amine, preferably trimethylamine. This process gives yields of approximately 50% of theory.
Kompolthy et al, Hungarian Patent T/9639 VE-719 (CO6f9/04) discloses a process for air oxidation of TNT to produce HNS in two steps as follows: ##EQU1##
They disclose yields of 82% HNBB and 76-91% HNS from HNBB by employing dimethylformamide or dimethylsulfoxide as solvents in a reaction mixture comprising methanol, potassium hydroxide, copper sulfate and pyridine. Efforts to repeat this procedure have only been successful in yielding 25-40% HNS.
Thus, there has been a need for a process for production of HNS in high reproducible yields.
In my copending U.S. Patent application, Ser. No. 069,216, filed Aug. 23, 1979, I have disclosed a process of producing good yields of HNS by reacting HNBB with copper sulfate in a reaction medium consisting essentially of hexamethylphosphoric triamide (HMPT) solvent. The process further discloses that the amount of copper sulfate used can be reduced much below stoichiometric amounts when a stream of an oxygen-containing gas, e.g., air is introduced into the reaction mixture. Substantially lower yields were obtained when the solvents N,N-dimethylformamide, N-methylpyrrolidinone, dimethylsulfoxide, pyridine and tetramethylurea were used in place of HMPT. Similarly, other copper and cobalt salts in hexamethylphosphoramide gave considerably lower yield of HNS than copper (II) sulfate, with copper acetate being only moderately effective (58% yield) while copper bromide, copper chloride, cobalt chloride, cobalt acetate and a cobalt ethylene-diamine-2,4-pentanedione complex gave poor yields.
My other copending U.S. Patent application, Ser. No. 020,881, filed Mar. 15, 1979, discloses and claims preparation of HNS by reacting HNBB with a copper ammino compound, e.g., copper tetraamminosulfate in a solvent including hexamethylphosphoramide (HMPT).
A process for converting HNBB to HNS through reaction of HNBB with a halogenating agent in the presence of a base and a suitable solvent is disclosed in copending U.S. Patent application, Ser. No. 020,889 by Everett E. Gilbert, filed Mar. 15, 1979. The halogenating agents include chlorine, bromine, iodine, N-halogen derivatives, e.g., N-chlorosuccinimide, N-bromosuccinimide and N-bromoacetamide, and hypohalites. The base used can be organic, e.g., tertiary amines such as pyridine and triethylamine or inorganic, e.g., sodium hydroxide, ammonium hydroxide, magnesium oxide, sodium carbonate and sodium bicarbonate. Acceptable yields of HNS were obtained with a pyridine base and the preferred bromine or N-bromosuccinimide halogenating agents.
Though prior methods have been successful in producing acceptable yields of HNS, there remains a need for a simplified process for producing HNS in high reproducible yields.